I have a 1985 Mustang LX 5.0 with 4bbl, 5 speed, 108kmi, and only drive it in the summer. Can't lower the curb idle w/o adjusting the timing. Can't get curb idle below 900RPM, easily. Any thoughts?

Background info, sorry in advance for writing a novel, but ....
About 6 months ago my car wouldn't start and figured out it is the pick-up coil in the distributor. Since replacing the coil required removing the distributor, I bought a rebuilt distributor exactly the same as mine. I put in the new distributor and "eye-balled" the timing by setting the distributor about half between rough idle and pinging, all was fine. Now, I knew the car needed to be inspected so I checked the timing and noticed my initial setting was 8deg BTDC and the spec is 10 BTDC. I incr the timging to 10 BTDC and noticed that the idle increased a little, I figured that was a good thing. The car is running great, I got 20MPG the last two tank fulls, not bad for a 30 year old car. The idle was about 1000 RPM, I knew that was a little high but took it to the DMV and it failed for high HC's at curb idle, 700 vs 200ppm. However, since fast idle emissions are fine, I think it is just related to the idle setting.

Now, I backed off the idle screw (drivers side) and can't lower the idle, I can only increase the idle. I remembered that the idle increased with timing incr from 8 to 10 deg.. So, I lowered the idle to 6 deg BTDC and now I have more control over the curb idle. I was able to set the idle below 750RPM. but ...... I notice when the car is sitting alongside the curb for a long time the idle drops to my setting of < 750RPM. However, when I drive it and stop at a stop-light or stop sign, the idle drops < 750RPM, but immediately climbs to >750RPM. Not quite as high as a 1000RPM, but it definitely doesn't remain <750RPM. I don't recall this happening in the past and I am concerned I will fail curb idle emissions again.

You car uses timing to help adjust the idle. You have to make sure it is installed correctly with #1 cylinder at compression TDC.

2014 V6 Deep Impact Blue Mustang, Borla Type S, 2.73 (Uughh need 3.73)1989 Sonic Blue Mustang LX Coupe, 349 ci, 392 rwhp 424 rwtq....Mustang Dyno"There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves." -Will Rogers-

yes it is, otherwise I would expect the car wouldn't run right at all. I re-installed the distributor with rotor pointing exactly where it was previously. In addition, I was able to set engine to 0 deg timing mark and the rotor was pointing at #1 plug.